Polymer electrolyte fuel cells using solid polymer membranes having proton conductivity operate at a lower temperature compared with other fuel cells, such as solid oxide fuel cells and molten carbonate fuel cells. Therefore, the polymer electrolyte fuel cells are receiving increased attention as a driving power source for use in moving bodies such as vehicles and have already been put to practical use.
A gas diffusion electrode used in such a polymer electrolyte fuel cell generally includes an electrode catalyst layer and a gas diffusion layer. The electrode catalyst layer contains catalyst-supporting carbon fine particles covered with ion exchange resin (a polymer electrolyte) identical to, or different from, a polymer electrolyte membrane. The gas diffusion layer functions to supply reactant gas to the catalyst layer and collect charges generated in the catalyst layer. Gas diffusion electrodes are assembled in a manner such that the catalyst layers face the polymer electrolyte membrane, thereby forming a membrane electrode assembly (MEA). Plural membrane electrode assemblies are stacked on top of each other via separators having gas passages, so as to compose a polymer electrolyte fuel cell.
There is known a gas diffusion layer used in such a polymer electrolyte fuel cell wherein a microporous layer, mainly containing an electrically conductive substance such as a carbon material, is provided as an intermediate layer in the gas diffusion layer on the catalyst layer side. The intermediate layer is provided to decrease electrical resistance between the gas diffusion layer and the catalyst layer and also improve gas flow therein. The gas diffusion layer is composed of the microporous layer together with a gas diffusion layer substrate. Therefore, in the gas diffusion layer, the microporous layer itself is also required to have high gas permeability in addition to high electrical conductivity.
For example, Patent Literature 1 discloses layers used in such a gas diffusion electrode, wherein the layers have a structure consisting of fine knot portions and fine fiber portions including PTFE resin and contain electrically conductive substance powder (carbon black). The layers hold therebetween a layer also consisting of fine knot portions and fine fiber portions including PTFE resin but not containing electrically conductive substance powder.